WEATHERING AND STABILIZATION OF POLYOLEFINS J. A. MELCHORE Organic Chemicals Division, American Cyanamid Co., Bound Brook, N . J.
Differences in the spectral characteristics of light sources influence the weatherability of polyolefins. The light sources used in this study were the Fade-Ometer, Weather-Ometer, fluorescent sunlamp-fluorescent blacklight, and outdoors. Differences in weatherability in Arizona were due to seasonal variations in ultraviolet content of sunlight and temperature. Polyethylene degradation was seven times more rapid in July than in December. A series of filters was used to determine the wavelengths most responsible for degradation (activation spectra) for polypropylene that did and did not contain ultraviolet absorbers. Polymer degradation was measured in terms of elongation, tensile strength, brittleness, and carbonyl formed (infrared). A new stabilizing system increases weatherability severalfold.
we have found it difficult to correlate artificial weathering data on polymers with outdoor data. Recently we have developed weathering data on polyolefins, using several exposure units. VER THE YEARS
Effect of light Source on Polymer Degradation
The ultraviolet output of light sources varies considerably (2, 5 ) . Hirt and coworkers determined the absolute spectral energies for various light sources ( 2 ) . Some of these data are plotted in Figure 1. The total intensity and energy distribution are different for sunlight and the various accelerated weathering devices. The Fade-Ometer and the Weather-Ometer are two devices most commonly used in aging studies. The Fade-Ometer has two intense energy peaks around 360 and 380 mp. The spectral output of the Weather-Ometer is similar to that of the Fade-Ometer, except that these two peaks are not so intense. The FadeOmeter, in particular, has been widely used in testing polyolefins. For some time we have been using a fluorescent sunlamp-fluorescent blacklight unit.
400 300
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\Ve have examined the degradation of polypropylene under several light sources. The resistance of unstabilized polypropylene to ultraviolet light was so poor that little difference in weatherability could be detected during exposure in the different weathering units. However. when polypropylene containing two of the most commonly used light stabilizers for polypropvlene was exposed, a reversal in the protection of polypropylene was noted in the Fade-Ometer and outdoors. Because of this anomaly, polypropylene films containing these two light stabilizers were exposed to other ultraviolet light sources to determine if a correlation could be established with "outdoor weathering" data (Figure 2). The two light stabilizers are different spectrally as well as chemically;
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1,186
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Ten 20-watt fluorescent sunlamp bulbs are alternately mounted with ten 20-watt fluorescent blacklight bulbs in a circular position. O n a predetermined staggered schedule, each bulb is progressively replaced after 2000 hours of service, which means that a t every 100-hour interval one bulb from this unit is replaced. This procedure minimizes any spectral irregularity in any one bulb. The samples are mounted vertically 3 inches from the lamps on a drum, which rotates about the circular bank of lamps.
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2-HYDROXY-4-OCTYLOXYBENZOPHENONE (.25%) 2-2'-THIOBIS (4-t-OCTYLPHENOL) 2:l N I C K E L COMPLEX (.25%)
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Energy output of several ultraviolet sources
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WOM FS-BL ARIZONA* FOM (55°C) (5OoC) (30°C) (5-33C) * MARCH-MAY Figure 2. Effect of light source on polymer embrittlement
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1 &mil unstobilized polypropylene film
232
I & E C PRODUCT RESEARCH AND DEVELOPMENT
2-hydroxy-4-octyloxybenzophenone is a very efficient ultraviolet absorber in the 300- to 400-mp range. whereas 2,2’thiobis(4-tert-octylphenol) 2 to 1 nickel complex absorbs very little in this ultraviolet range and has been referred to in the literature as a “reactive light stabilizer” (7). The polymer film was rated as “brittle” when it could no longer be folded back on itself (180’) without cracking. \Ye repeated the above comparison, but added these light stabilizers to a commercial polypropylene that contained a n antioxidant (Figure 3). Again the weathering data from the Fade-Ometer did not correlate well with outdoor aging. The data from the Weather-Ometer were in better agreement with outdoor data than the data from the Fade-Ometer, because the Weather-Ometer (Sunshine -4rc WeatherOmeter Model XW) has a spectral output somewhat closer to sunlight. The data from the fluorescent sunlamp-fluorescent blacklight unit gave the best correlation, since this light source has the closest spectra to sunlight below 350 mp. Thus data on the protection derived from light stabilizers when exposed to artificial light can be misleading, unless a weathering correlation has been established between artificial light and outdoors data.
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Effect of light source on polymer embrittlement
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Effect of Wavelength on Polymer Degradation
The difference observed in light stability of polypropylene exposed in the different exposure units suggested that we determine the wavelengths most responsible for the degradation of polypropylene-i.e., activation spectrum of the polymer. Hirt (3) has determined the activation spectrum for polypropylene by measuring the change in ultraviolet absorption characteristics of the polymer when exposed to spectrally dispersed radiation from a filtered xenon source. I n these and subsequent experiments, he found the greatest change in ultraviolet absorbance, and presumably most polymer degradation, a t two wavelengths: 297 and 370 my. I n this investigation. a somewhat different technique was employed, so that activation spectra could be determined on much larger samples; polymer degradation was now followed by change of physical properties-elongation, tensile strength, and brittleness-and also by the formation of carbonyl, using infrared ( 3 ) . A series of glass ultraviolet absorbing filters (2 X 2 inches) was used in this study to determine activation spectrum. The transmittance characteristics of these filters are shown in Figure 4. T h e effect of exposing unstabilized polypropylene behind this series of filters, which cuts out certain regions of the ultraviolet, is noted in Figure 5. T h e greatest polymer degradation, as expected, occurred in the sample not protected by a filter; less degradation occurred as the ultraviolet screening efficiency of the filters increased.
Table I.
Effect of Ultraviolet Cutoff Point on Polymer Properties
Exposed behind filters for 59 hours in FS-BL unit Properties of Film after Exposure cutoff Point of Filter, M P
364 347 313 303 277
70.
5% carbonvl formed 0.00 0.00 0.02 0.07 0.10
Brittle No No
No No Yes
tensile strength retained 93 88 88 64 37
70
elongation retlined 88 76 55 33 12
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WAVELENGTH ( r y ) Transmittance of filters
I n determining activation spectrum, 2 X 2 X 0.015 inch samples of polypropylene were placed l / 8 inch in back of the filters, so as to minimize heat effects. The samples were exposed in a Fade-Ometer as well as in a fluorescent sunlampfluorescent blacklight unit until incipient degradation was detected by carbonyl formed. The greatest change in polymer properties, when exposed in the fluorescent sunlamp-fluorescent blacklight, occurred at about 305 mp (Table I). When the Fade-Ometer was used, the activation spectrum was not as sharply defined (Table 11). A rapid loss in elongation occurred with little carbonyl formed. I t is suspected that the Fade-Ometer’s unrealistically high amount of energy in the 357- to 390-rnp range is responsible for so great a loss in elongation with so little carbonyl formed. If polymer embrittlement and carbonyl formed are used as measures of polymer degradation, most polymer degradation is caused by wavelengths a t about 300 mp.
Table II.
Effect of Ultraviolet Cutoff Point on Polymer Properties Exposed behind filters for 43 hours in Fade-Ometer Properties of Film after Exposure cutoff 70 Point of % tensile 70 Filter, carbonyl strength elongation il!f/.4 formed Brittle retained retained 364 0 02 N on 7 _ n_ 347 0 03 N O 76 7
313 303 277
0 03 0 03 0 07
VOL. 1
NO
Slight Yes
NO. 4
65 58 42
6 5 4
DECEMBER 1962
233
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HOURS EXPOSED (FLUORESCENT SUNLAMP) Figure 5.
Relation of time to polymer degradation Various filters
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12 16 20 24 28 32 HOURS TO EMBRITTLE POLYMER Figure 6. Effect of temperature of film during degradation on degradation time 4
Hanovia Hg lamp with Corex D tllter 15-mil polypropylene tllm
Effect of Temperature on Photodegradation
I n exposure tests with polyolefins, we found that the ambient temperature influenced the rate of polymer degradation. The over-all reaction during photodegradation is temperaturedependent and is obviously not a zero-order reaction, as it would be if it were governed solely by light intensity. I n determining the effect of temperature change during ultraviolet exposure on the rate of polymer degradation, the following experiment was run.
A 11/2-inch-diameter polypropylene film (1 5 mils) was placed in a 0.5-inch recessed reservoir of a 3-inch-diameter aluminum cylinder, electrically heated. The sample reservoir was covered with a Corex D filter and the temperature within was measured with a thermometer inserted through a hole drilled in the side of the cylinder. A Hanovia mercury lamp was placed 4 inches directly above the Corex D filter and the time a t which the polymer became brittle was noted. These data are plotted in Figure 6. A 10' C. increase in exposure temperature about doubled the rate of degradation. Therefore, when exposure conditions involve different and/or fluctuating temperatures, correlation of weathering data will be poor. Seasonal Weathering
Realizing that exposure outdoors in the summer is more severe than in the \vinter, we decided to see if this could be more quantitatively determined over a year period. Polymer degradation has often been measured in terms of langleys incident upon the sample. The langley. defined as a gram calorie per square centimeter, measures total radiation, which normally consists of 50% visible, 45% infrared? and 5% ultraviolet. In this study with polyethylene, film \vas exposed for each calendar month over a 1-year period. The film degraded, as measured by carbonyl formed and change in elongation, about seven times more rapidly in the summer months than in the winter months (Figure 7). I n a concurrent experiment, film was exposed for 15,000 langley-s, starting at the first of each month. Depending upon weather conditions, 15,000 langleys were reached between a 26- and 32-day period; 15.000 langleys caused about seven times more degradation in the summer than in the winter (Figure 7'). Thus the langley is a poor unit to measure ultraviolet radiation. A very important variable that contributes to the greater degradation in the summer is the increase of ultraviolet light having wavelengths less than 313 mp (2, 4) (Figure 8). Hirt, Searle, and Schmitt established the activation spectrum for 234
l & E C PRODUCT RESEARCH A N D
DEVELOPMENT
polyethylene to be about 300 mp; therefore, this enrichment of low ultraviolet energy in the summer can contribute toward faster polymer degradation ( 3 ) . 4 n increase in temperature during irradiation increases polymer degradation. The mean monthly temperature curve for -4rizona followed closely the degradation curve for the year. On the bases of responses noted in Figure 8, variations in polymer degradation from season to season can be attributed to seasonal fluctuations in spectral distribution of ultraviolet energy as well as to changes in temperature. Ultraviolet Stabilization of Polyolefins
Some of the unstabilized polypropylene, which embrittled after only 59 hours in the fluorescent sunlamp-fluorescent blacklight unit. was now light-stabilized by the addition of 0.25Yc 2-hydroxy-4-octyloxybenzophenone.The light stabilizer extended the embrittlement time from 59 hours to 1079 hours. The activation spectrum peak of this ultravioletstabilized polypropylene was now observed to fall in the region of 293 mp (Table I I I ) ? which is about 12 mp lower than for polymer ivithout light stabilizer. Polypropylene has a very high absorption coefficient for the short wavelengths, which may not be filtered out by a light stabilizer. Therefore, the activation spectrum peak for ultraviolet-stabilized polymer shifted to loiver ivavelengths. In Figure 9 we note that the lower and higher ivavelengths not strongly absorbed by the light stabilizer finally degraded the polymer after very prolonged exposure. In fact, if one plots the activation spectra for unstabilized and ultraviolet-stabilized polypropylenes and superimposrs the ultraviolet transmittance curve of the light stabilizer, one can see that the light stabilizer very efficiently screened out the most deleterious wavelengths (Figure 9).
Table 111.
Effect of Ultraviolet Cutoff Point on Polymer Properties Exposed behind filters for 1079 hours in FS-BL Properties of Film after Exposure
cutoff Point of Filter, Alp
c /o carbonyl formed
364 347 31 3 303 277
0.02 0.03 0 03 0.03 0.12
7.
NO
tensile strength retained 97
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88
N O
93 75 21
Brittle
NO Yes
%
elongation retained 9 7 7 6 2
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Light stabilizers probably protect polypropylene by different mechanisms. TVe have found that 2-hydroxy-4-octyloxybenzophenone protects mainly by ultraviolet absorption, although it exhibits some antioxidant properties as measured during heating in a 90’ C. oven. The 2,2’-thiobis(4-tertoctylphenol) 2 to 1 nickel complex absorbs very little ultraviolet energy in the 300- to 400-mp region and our studies have found it to exhibit good antioxidant properties. This suggested that each stabilizer could be functioning independently and that a combination of these two types of light stabilizers might be very desirable. Other authors ( 7 ) have recently expressed similar vieivs. TVhen \
